This invention relates to transistor test fixtures used in testing RF transistors in microwave frequencies (see ref. 1, 2). Active RF components (transistors—DUT) need to be thoroughly tested at the operation frequencies before used in amplifier and other circuit designs. “Load pull” and “Source pull” are test methods which use impedance tuners to systematically characterize the DUTs under various load and source impedance conditions. Load pull or source pull are automated measurement techniques used to measure Gain, Power, Efficiency and other characteristics of the DUT, employing source and load impedance tuners and other test equipment, such as signal sources, directional couplers, test fixtures to house the DUT (device under test, typically an RF transistor) and input and output power meters (FIG. 1), see ref. 3.
At high power the transistors become “non-linear”, i.e. input and output signals are not any more directly proportional. A sinusoidal signal at the input is deformed at the output, meaning that it contains “harmonic components”. Non-sinusoidal periodical signals in the “time domain” can be described in the “frequency domain” as a Fourier series of the fundamental and harmonic frequency components using a Fourier transformation, see ref. 4 and FIG. 12. An “Inverse” Fourier transformation allows reversing the presentation of the signals from the frequency domain back to the time domain. Most RF instruments operate in the frequency domain, i.e. they process the various frequency components of an incoming signal at their input (test) ports. In order to be able to observe the actual signal waveform at the DUT ports, whereas the measurement occurs at a different position in the network, we must work in the “frequency domain”, using the fundamental and harmonic components generated by the Fourier transformation (see ref. 4).
It is obvious that the higher the number of harmonics considered, the more accurate the description of the time behavior of the original signal (FIG. 12) becomes. This means, however that, as the harmonic components traverse the network (most passive networks have “low pass” behavior) higher harmonic components will be attenuated more than lower ones which means that the original signal is “smoothed” out. In other words, the signal form changes as the (non-sinusoidal) signal travels through a passive network such as a transmission line. By cascading (“embedding”) the signal harmonic components with the transfer matrix between the measurement point and the DUT will reconstitute the original signal form. However parasitic components, loss and low pass behavior of the network reduce the higher harmonic components significantly (FIG. 14); then reconstitution of the original signal form is affected negatively. This can be seen in FIG. 4: True reconstitution of the original signal form between the measurement reference plane (48) and the DUT port (49) requires very high accuracy both of the measurement at the deferred position (48) and the transfer matrix of the transformation section (415) between the test reference plane (48) and the DUT (49). This is the typical problem in analog telephony and associated distortion of the higher tones in voice or music transmission using long cables between the amplifier and the speakers. Therefore, the closer the actual measurement to the DUT occurs, the easier and more accurate will be the reconstitution of the original signal waveform.
At high frequencies electronic equipment, such as signal and network analyzers, operate in the frequency domain. In the frequency domain it is also easy to shift the reference plane of the measurement. In order to sample the signal's components over a wide frequency range (the more harmonic components are known, the more accurate is the correspondence between the time and frequency domain of a signal) we need wideband signal couplers (see ref. 5). Those couplers can be connected on both sides of the test fixture in which the DUT is mounted (FIGS. 1, 2). However such a setup has insertion loss and signal deformation due to parasitic components of the connectors between couplers and test fixture and transmission lines inside the fixtures and is therefore not an optimum embodiment.